It is well known in the art that an image located on an input sample, such as a transparency, may be read by a specific family of optical scanners. A scanner of this family generally comprises a light source, a lens, an apparatus for holding the input sample, and a detector array. The input sample is often held in a cassette comprised of two glass plates.
It is important that the output of the scanner be uniform in the presence of a uniform input. Non-uniform scanner output often occurs because individual detectors in the detector array have different sensitivities to light and, therefore, do not output identically even while receiving identical inputs. This problem is corrected by normalization, i.e., by multiplying each detector's output by a suitable factor to produce a normalized output, such that each of the normalized outputs is identical.
Non-uniform scanner output may also result from a variety of non-uniformities of the light source. For example, a fluorescent lamp light source might have a scratch or a non-uniform phosphor coating on its glass tube, or a light source employing fiber optic bundles might have non-uniform bundle packaging or a break in one or more fibers.
Non-uniform scanner output results in non-uniform images of the input sample. For example, scratches on the glass of the light source cause abrupt changes in light intensity which, in turn, causes thin or sharp stripes in the output image. "Slow" changes in light intensity (changes which vary slowly as a function of the spatial position on the lamp) cause wide or smooth stripes in the output image.
Thin or sharp stripes in the image are the most undesirable whereas wide or smooth stripes are often difficult or impossible for an observer to notice.
As the sharp non-uniformity is undesirable, the light source non-uniformity is compensated in a number of ways. The cassette can be moved until a portion of the cassette in which there is no transparency is in the optical axis. The light distribution on the detector is then measured to establish correction factors.
However, the optical quality of the cassette is rarely consistent throughout the cassette. Thus, the light distribution during scanning can be different than that during calibration which occurrence will produce non-uniform output images.
Another way to solve the light non-uniformity problem is to remove the cassette during calibration and thus, to ensure that the inconsistencies of the cassette do not compound the light uniformity error. However, in this solution, the optical system for calibration is significantly different than that for scanning. The image of the light source on the detector with the cassette on an optical axis of the scanner is larger than it is without the cassette and therefore, the light non-uniformity sensed by the detector is different during scanning than during calibration.